During the deposition of materials on a semiconductor wafer, it is desirable to prevent materials from depositing on the edge of the front surface, on the end edges and on the backside of the wafer. This is important when the wafer requires surface treatment to improve the adhesion of the deposited material as in the case of tungsten deposition. The wafer surface needs to be coated with an adhesion promoter material such as titanium tungsten (TiW), or titanium nitride (TiN) before the deposition of tungsten to ensure proper adhesion. When tungsten is deposited on the front edge, on end edges or on backside of the wafer where there is no TiW or TiN, the deposited tungsten does not adhere properly and can flake off as particles. The generation of particles such as these could be damaging to subsequent wafer processing. Edge and backside exclusion is also of particular importance when the deposited materials require a diffusion barrier layer to prevent the deposited materials from reaching the silicon wafer, which can create device degradation. For example, copper can be deposited on a diffusion barrier layer such as TiN, tantalum nitride, or tungsten nitride. Without the diffusion barrier layer, the copper could migrate to the silicon area and lower device performance. Deposition of copper on the backside, on the end edges or on the front edge where there is no diffusion barrier material severely affects device properties.
FIG. 1 shows a prior art edge exclusion apparatus employing purging gas to prevent edge and backside deposition. Deposition precursor enters the inlet 20, and deposits on the wafer 10. The inlet 20 could be a showerhead, providing precursor flow 16 to the wafer 10 at a more uniform distribution. Purging gas 15 enters the gap between the wafer holder 30 and the blocker 24 to prevent material deposition at the wafer 10 edge and backside. Precursor flow 16 continues to 26 and purging gas 15 continues to 25 to reach the exhaust. The major drawback of this prior art apparatus is the high purging gas flow rate required to prevent edge and backside deposition, which is typically in the range of liter per minute flow. Therefore, this apparatus is not suitable for a system using low precursor flow.
Another prior art apparatus as disclosed in U.S. Pat. No. 4,932,358 to Studley et al. includes a seal ring which presses down against a wafer that is on a CVD chuck. The seal ring presses continuously around the outer periphery of the wafer. Sufficient force is applied to hold the backside of the wafer against the chuck. This apparatus requires a complicated mounting mechanism to move the seal ring in and out of clamping engagement with the wafer and to maintain alignment between the seal ring and the wafer. Furthermore, the seal ring can only be as wide as the diameter of the chuck.
FIG. 2 shows a prior art apparatus from U.S. Pat. No. 5,851,299 to Cheng et al. which includes a shield ring 50 that normally rests on a ring support 72. The shield ring 50 engages the front side edge of the wafer 10 when the wafer support 40 is raised into the contact position by the susceptor lift 46. The wafer edge and backside are shielded from the precursor flow from the showerhead 20. Cheng et al. also discloses an additional purging gas flow 1 which is retained in the cavity between the wafer support 40, the wafer 10 and the shield ring 50. The purging gas exhausts through the gap 2 between the ring support 72 and the shield ring 50, and combines with the precursor exhaust 3 to reach the vacuum pump.
As with the other prior art, the major drawback of this shield ring is that eventually there will be some deposition at the edge of the shield ring at the locations where the shield ring contacts the wafer. The gap between the shield ring and the wafer, which is caused by material deposition widens over time. This process causes the shield ring to lose contact with the wafer so that the shield ring no longer performs the shielding function. The apparatus will need to be shut down, the chamber vented, and the shield ring manually replaced. The chamber will have to be pumped down and the system will have to be conditioned for process qualification before operating again. This procedure causes a significant [lost] loss in productivity.
The purging gas is helpful in reducing the build up of material deposit at the shield ring edge. However in the prior art Cheng et al. apparatus, as seen in FIG. 2, the purging gas can easily escape through the big gap between the shield ring 50 and the ring support 72. In the Cheng et al. apparatus, this gap is required for proper shielding of the wafer. The minimum gap size is probably 0.1″ to allow adequate separation between the shield ring and the wafer for the removal of the wafer. Assuming a 10″ diameter for the shield ring for the processing of an 8″ wafer, the purging gas area is 0.1×10, which translates into an equivalent diameter D of 1.1″. The 1.1″ diameter opening would require a very high flow rate in order to retain the purging gas at the connection of the wafer and the shield ring to prevent material deposition at that location, especially when the typical inlet of the purging gas is only 0.25″ in diameter.
Another major draw back of the prior art apparatus is the uniform temperature profile of the shield ring in high temperature processing systems. In these systems, the wafer is heated for the process reaction to take place, but it is desirable to have the shield ring cooler than the wafer to prevent reaction at the shield ring. Using high thermal conductivity materials will raise the temperature of the shield ring through the transfer of thermal energy from the heated wafer. Using low thermal conductivity materials will lower the temperature of the shield ring, but the wafer temperature will no longer be uniform because of the heat loss at the contact area caused by the cool shield ring. Using a high thermal reflectivity material would solve this problem because all the heat would be reflected back, and the shield would be cooled without draining the thermal energy from the wafer edge. However, there is currently no effective high thermal reflectivity material available.
It would be advantageous to develop a shielding apparatus that has a variable temperature profile.
It would be advantageous to develop a shielding apparatus that does not cause the heat loss at the wafer edge.
It would be advantageous to develop a shielding apparatus that reduces the down time of the system.
It would be advantageous to develop an apparatus with a smaller purging gas escape flow.